Numerous methods and systems have been developed for the detection and quantitation of analytes of interest in biochemical and biological substances. Methods and systems which are capable of measuring trace amounts of microorganisms, pharmaceuticals, hormones, viruses, antibodies, nucleic acids and other proteins are of great value to researchers and clinicians.
A very substantial body of art has been developed based upon the well known binding reactions, e.g., antigen-antibody reactions, nucleic acid hybridization techniques, and protein-ligand systems. The high degree of specificity in many biochemical and biological binding systems has led to many assay methods and systems of value in research and diagnostics. Typically, the existence of an analyte of interest is indicated by the presence or absence of an observable "label" attached to one or more of the binding materials. Of particular interest are labels which can be made to luminesce through photochemical, chemical, and electrochemical means. "Photoluminescence" is the process whereby a material is induced to luminesce when it absorbs electromagnetic radiation. Fluorescence and phosphorescence are types of photoluminescence. "Chemiluminescent" processes entail the creation of luminescent species by chemical transfer of energy. "Electrochemiluminescence" entails creation of luminescent species electrochemically.
Chemiluminescent assay techniques where a sample containing an analyte of interest is mixed with a reactant labeled with a chemiluminescent label have been developed. The reactive mixture is incubated and some portion of the labeled reactant binds to the analyte. After incubation, the bound and unbound fractions of the mixture are separated and the concentration of the label in either or both fractions can be determined by chemiluminescent techniques. The level of chemiluminescence determined in one or both fractions indicates the amount of analyte of interest in the biological sample.
Electrochemiluminescent (ECL) assay techniques are an improvement on chemiluminescent techniques. They provide a sensitive and precise measurement of the presence and concentration of an analyte of interest. In such techniques, the incubated sample is exposed to a voltammetric working electrode in order to trigger luminescence. In the proper chemical environment, such electrochemiluminescence is triggered by a voltage impressed on the working electrode at a particular time and in a particular manner. The light produced by the label is measured and indicates the presence or quantity of the analyte. For a fuller description of such ECL techniques, reference is made to PCT published application US85/01253 (W086/02734), PCT published application number US87/00987, and PCT published application U.S. 88/03947. The disclosures of the aforesaid applications are incorporated by reference.
It is desirable to carry out electrochemiluminescent assays without the need for a separation step during the assay procedure and to maximize the signal modulation at different concentrations of analyte so that precise and sensitive measurements can be made. Among prior art methods for nonseparation assays are those which employ microparticulate matter suspended in the assay sample to bind one or more of the binding components of the assay.
U.S. Pat. No. 4,305,925 relates to the detection and determination of clinically relevant proteins and peptides by means of nephelometric and turbidimetric methods. The methods disclosed involve binding the antigen or antibody to latex particles which perform the function of light scattering or adsorption.
U.S. Pat. No. 4,480,042 relates to techniques employing particle reagents consisting of shell-core particles. The shell contains functional groups to which compounds of biological interest can be covalently bonded, and the high refractive index of the core results in high sensitivity to light scattering measurements. The technique is based upon agglutination reactions which result from the reaction of bivalent antibodies with multivalent antigens of interest to produce aggregates which can be detected and/or measured in various ways.
U.S. Pat. No. 4,419,453 likewise relates to the use of colored latex agglutination test methods useful for detecting the presence of immunochemicals such as antibodies and immunogens.
Based upon this prior art, it would not have appeared possible to use microparticulate matter in assays wherein a luminescent phenomenon is measured. One would expect that the luminescence from free chemiluminescent or electrochemiluminescent moieties would be absorbed, scattered, or otherwise suffer interference from the microparticulate matter.
Contrary to that expectation, U.S. application Ser. No. 539,389, now abandoned, (PCT published application U.S. 89/04919) teaches sensitive, specific binding assay methods based on a luminescent phenomenon wherein inert microparticulate matter is specifically bound to one of the binding reactants of the assay system. The assays may be performed in a heterogeneous (one or more separation steps) assay format and may be used most advantageously in a homogeneous (nonseparation) assay format.
U.S. 89/04919 relates to a composition for an assay based upon a binding reaction for the measurement of luminescent phenomenon, which composition includes a plurality of suspended particles having a surface capable of binding to a component of the assay mixture. In another aspect, it is directed to a system for detecting or quantitating an analyte of interest in a sample, which system is capable of conducting the assay methods using the assay compositions of the inventions. The system includes means for inducing the label compound in the assay medium to luminesce, and means for measuring the luminescence to detect the presence of the analyte of interest in the sample.
It was found that the binding of that component of the assay system to which an electrochemiluminescent moiety has been linked, to suspended microparticulate matter, greatly modulates the intensity of the luminescent signal generated by the electrochemiluminescent moiety linked to that component, thereby providing a means of monitoring the specific binding reaction of the assay system. Even more surprisingly, the suspended particles were found to have little or no effect on the intensity of the luminescent signal generated by the electrochemiluminescent moiety linked to the component of the system which remains unbound to the suspended microparticulate matter.
Thus, U.S. 89/04919 is directed to methods for the detection of an analyte of interest in a sample, which method includes the steps of (1) forming a composition comprising (a) a sample suspected of containing an analyte of interest, (b) an assay-performance-substance selected from the group consisting of (i) analyte of interest or analog of the analyte of interest, (ii) a binding partner of the analyte of interest or its said analog, and (iii) a reactive component capable of binding with (i) or (ii), wherein one of said substances is linked to a label compound having a chemical moiety capable of being induced to luminesce, and (c) a plurality of suspended particles capable of specifically binding with the analyte and/or a substance defined in (b)(i), (ii), or (iii); (2) incubating the composition to form a complex which includes a particle and said label compound; (3) inducing the label compound to luminesce; and (4) measuring the luminescence emitted by the composition to detect the presence of the analyte of interest in the sample. Those same methods may be used to quantify the amount of analyte in a sample by comparing the luminescence of the assay composition to the luminescence of a composition containing a known amount of analyte.
Analogs of the analyte of interest, which may be natural or synthetic, are compounds which have binding properties comparable to the analyte, but include compounds of higher or lower binding capability as well. Binding partners suitable for use in the present invention are well-known. Examples are antibodies, enzymes, nucleic acids, lectins, cofactors and receptors. The reactive components capable of binding with the analyte or its analog and/or with a binding partner thereof may be a second antibody or a protein such as Protein A or Protein G or may be avidin or biotin or another component known in the art to enter into binding reactions.
Advantageously, the luminescence arises from electrochemiluminescence (ECL) induced by exposing the label compound, whether bound or unbound to specific binding partners, to a voltammetric working electrode. The ECL reactive mixture is controllably triggered to emit light by a voltage impressed on the working electrode at a particular time and in a particular manner to generate light. Although the emission of visible light is an advantageous feature the composition or system may emit other types of electromagnetic radiation, such as infrared or ultraviolet light, X-rays, microwaves, etc. Use of the terms "electrochemiluminescence," "electrochemiluminescent," "luminescence," "luminescent," and "luminesce" includes the emission of light and other forms of electromagnetic radiation.
The methods taught in U.S. 89/04919 permit the detection and quantitation of extremely small quantities of analytes in a variety of assays performed in research and clinical settings. The demands of researchers and clinicians makes it imperative, however, to lower the detection limits of assays performed by these methods to increase the sensitivities of those assays and to increase the speed at which they can be performed.
Various methods are known in the art for increasing the signal from labeled species by concentrating them before subjecting them to a measurement step. In U.S. Pat. No. 4,652,333, for example, particles labeled with fluorescent, phosphorescent or atomic fluorescent labels are concentrated by microfiltration before a measurement step is performed.
It is also known in the art to concentrate labeled immunochemical species prior to a measurement step, by, e.g., drawing magnetically responsive labeled particles to the surface of a measurement vessel. In U.S. Pat. Nos. 4,731,337, 4,777,145, and 4,115,535, for example, such particles are drawn to the vessel wall and then are irradiated to excite a fluorophoric emission of light.
In U.S. Pat. No. 4,945,045, particles are concentrated on a magnetic electrode. An electrochemical reaction takes place at the electrode facilitated by a labeled chemical mediator. The immunochemical binding reaction alters the efficiency of the mediator resulting in a modulated signal when binding takes place.
These prior art methods are not relevant to the surface selective excitation processes of the invention. While not being bound by any particular mechanistic explanation of surface excitation, e.g., electrochemiluminescence, it is believed that the label on the solid-phase complex must be oxidized at the electrode. This requires that an electron move from the label to the electrode. It is believed that the electron makes this "jump" by a phenomenon known as tunneling in which the electron passes through spice (a region where its potential energy is very high, e.g., the solution) without having to go "over" the potential energy barrier. It can tunnel through the energy barrier, and thus, move from one molecule to another or from one molecule to an electrode without additional energy input. However, this tunneling phenomenon can only operate for very short distances. The probability of the tunneling phenomenon falls off exponentially as the distance between the two species increases. The probability of the tunneling phenomenon occurring between two species is fairly high if the distance is less than 25 Angstroms (2.5 nm) but is fairly low if the distance is greater. The distance of 25 .ANG. is a rule-of-thumb used by those skilled in the art but is not an absolute limitation.
Accordingly, only those ECL labels with 25 .ANG. of the surface of the electrode can be expected to participate in the ECL process. The area of the particle which is within 25 .ANG. of the surface of an electrode is typically extremely small.
Accordingly, one would not expect that ECL from a particle surface would be measurable to any significant degree. Moreover, the light which is produced by the ECL process must pass through the particle to get to the photomultiplier. Since the particles are essentially opaque (a concentrated suspension of them is black) one would not expect that, even if significant amounts of light could be produced by ECL, that the light could pass through the particle and be measured by the photomultiplier.